This is a continuation of co-pending U.S. Pat. Application Ser. No. 423,805, filed Dec. 11, 1975 now abandoned.
In many industrial applications there is a need for accurately controlling the travel extent and velocity of a member driven along a predefined path. Industrial usages requiring such operation include virtually all operations where a work performing member is driven along a carriage. The work performing member may be called upon to perform a particular function during the travel period such as, for example, the grinding of a rotating piece of work stock by moving the grinder in reciprocating fashion the length of the stock. Alternatively, the work performing member may merely be driven between selected positions along the work piece and a particular function performed by a working tool at each position. This type of operation could include, for example, spot welding or drilling. In either event, there is a need for accurate control of the relative movement between the work piece and the work performing element. Accordingly, it is necessary to start and stop the driven member at precise locations and to decelerate the driven member at appropriate points to assure that the desired stop point is not overshot. For this reason, virtually all prior art systems accelerate the driven element as a selected rate until a maximum velocity of relative motion is attained. The relative velocity then remains constant until a selected location is reached at which time the moving member is decelerated until a minimum velocity is reached. Motion continues at the minimum velocity until the desired stop point is reached. The acceleration phase is used so that the driven element is gradually brought up to speed without undue strain on the system while overcoming the inertia of the driven member. The constant speed phase is selected in accordance with the maximum speed permissible for the particular system and operation to increase efficiency. The deceleration phase is used to gradually stop the driven element to prevent overshoot and thus increase the accuracy of the system.
In the oldest of prior art systems the velocity changes were effected by causing the driven member to physically contact limit switches. The limit switches actuated control circuitry to effect the velocity changes. Thus a limit switch was located at the position where deceleration, for example, was to occur. When the driven member contacted the switch, the deceleration was effected. Because limit switches have limited accuracy and high maintenance cost there have been many attempts to achieve the same control functions while eliminating the limit switches. Typically, systems eliminating limit switches utilize an electric motor and employ a control circuit for changing the speed of the electric motor. Such a system is described in U.S. Pat. No. 3,470,431 where a set of potentiometers establish reference voltages to actuate relays as the reference voltages and a voltage proportional to the motor position compare. The relays change the resistance of a D.C. motor circuit resulting in the desired speed changes of the motor.
Because the system described in the above referred system utilizes a DC electric motor, speed changes can be effected by changing the resistance of the motor circuit. However, in applications utilizing a hydraulic or mechanical drive mechanism other means must be provided for varying the speed of the driven element because speed changes of the hydraulic pump ordinarily cannot be effected simply by switching resistors in and out of the motor circuit. Accordingly, speed changes must be effected by changing the fluid flow from the hydraulic pump to the driven element. The instant element is directed to such a system.